The yellow leaves will not turn green again so it is best to trim them at the base with sharp scissors. I can't tell from your image but the crown of the plant should not be covered. Rather than disturb the plant, you can take a clean (new) turkey baster and gently blow the substrate away and expose the crown. Try using root tabs to fertilize because they help. Your aquarium looks new, are there any fish in the tank? There is much information on this site about plants so you might search there too.